1. Field of the Invention
The present invention relates to a voltage-controlled reactive power controller for a reactive power output from a self-commutated inverter connected to an AC system power source by controlling an output AC voltage of the self-commutated inverter.
2 Description of the Prior Art
In FIG. 1, there is shown a conventional voltage-controlled reactive power controller comprising an inverter main circuit including an inverter 10 having controllable reactifier elements and a DC capacitor 20 connected thereto in parallel, a coupling reactor 30, a coupling transformer 40, an AC system power source 100 connected to the inverter 10 through the reactor 30 and the transformer 40, a potential transformer, hereinafter referred to as PT, 61 for detecting an AC system voltage to output a system voltage signal, a current transformer, hereinafter referred to as CT, 62 for detecting an AC system current to output a system current signal, a reactive power detector, hereinafter referred to as Q-detector, 63 for detecting a reactive power of the AC system to output a reactive power signal Q.sub.F, a reactive power reference settler, hereinafter referred to as Q-reference settler, 81 for outputting a reactive power reference signal Q.sub.R, an adder 83 for operating an addition of the signal Q.sub.F and the signal Q.sub.R, a control compensator 84 for conducting a control compensation operation such as a proportional integration to obtain a phase difference signal .phi., and a phase controller 85 for operating firing timings of firing pulses for the rectifier elements of the inverter 10 so that the phase difference angle between an output voltage V.sub.IN of the inverter 10 and a system voltage V.sub.SY of the AC system power source 100 may be .phi., to output signals for instructing ON-periods to the rectifier elements of the inverter 10.
A principle of the operation of the conventional reactive power controller shown in FIG. 1 will now be described in connection with FIG. 2. In this case, a conventional reactive power control of a combination of an inverter main circuit 1 including the inverter 10 and the DC capacitor 20 with the AC system power source 100 via the reactor 30 and the transformer 40 is carried out as follows. It is considered that the inverter main circuit 1 is a power source for generating a reactive power Q. In FIG. 2, in fact, the inverter main circuit 1 includes the inverter 10 and the DC capacitor 20, and a coupling impedance 2 having an impedance X represents a combination of the reactor 30 and the transformer 40 because both the reactor 30 and the transformer 40 are considered as impedances.
In FIG. 2a, a current i flows between the inverter main circuit 1 and the system power source 100 through the impedance 2, and the phase difference angle .phi. exists between the system voltage V.sub.SY and the inverter output voltage V.sub.IN. FIG. 2b is a vector diagram when the inverter main circuit 1 acts as a reactor. The amplitude of the inverter output voltage V.sub.IN is smaller than that of the system voltage V.sub.SY, and a voltage V.sub.SY -V.sub.IN is applied to the impedance 2. The current i having the same phase component as that of the system power source 100 and a phase lag component of 90.degree. with reference to that of the system power source 100, flows through the impedance 2. This means that the inverter main circuit 1 acts as the reactor and receives an active power P from the system power source 100. This relation can be expressed in the following formulas: ##EQU1## That is, when the amplitude of the inverter output voltage V.sub.IN is smaller than that of the system voltage V.sub.SY and the phase of the inverter output voltage V.sub.IN is delayed with respect to that of the system voltage V.sub.SY, the inverter main circuit 1 functions as the reactor and receives the active power P from the system power source 100. As apparent from formulas (1) and (2), when the amplitude of the inverter output voltage V.sub.IN is smaller than that of the system voltage V.sub.SY and the phase of the inverter output voltage V.sub.IN is leading with respect to than that of the system voltage V.sub.SY, the inverter main circuit 1 spends the delayed reactive power Q and outputs the active power P to the system power source 100.
In formula (1), when the phase of the inverter output voltage V.sub.IN is leading that of the system voltage V.sub.SY, the phase difference angle .phi. is a positive value and, in turn, when the phase of the inverter output voltage V.sub.IN is lagging that of the system voltage V.sub.SY, the phase difference angle .phi. is a negative value. When the active power P is a positive value, the active power P is supplied from the inverter main circuit 1 to the system power source 100, and, in turn, when the active power P is a negative value, the active power P is supplied from the system power source 100 to the inverter main circuit 1. In formula (2), when the reactive power Q is a positive value, the inverter main circuit 1 acts as a capacitor, and, in turn, when the reactive power Q is a negative value, the inverter main circuit 1 functions as a reactor.
FIG. 2c is a vector diagram when the amplitude of the inverter output voltage V.sub.IN with reference to that of the system voltage V.sub.SY satisfies the following formula: ##EQU2## In this case, the current i having the same phase component as that of the system power source 100 and a phase lead component of 90.degree. with reference to that of the system power source 100, flows through the impedance 2. This means that the inverter main circuit 1 functions as the capacitor and receives the active power P from the system power source 100. Even though formula (3) is satisfied, as is clear from formulas (1) and (2), when the phase difference angle .phi. is a positive value, the inverter main circuit 1 acts as the capacitor and supplies the active power P to the system power source 100.
Then, the operation of the conventional reactive power controller for controlling a reactive power Q supplied to the system power source 100, as shown in FIG. 1, will now be described on the basis of the principle described above in connection with FIG. 2.
The reactive power supplied to the system power source 100 is detected as the reactive power signal Q.sub.F output from the Q-detector 63 which receives the system voltage signal and the system current signal from the PT 61 and the CT 62 and operates the reactive power. The reactive power signal Q.sub.F is fed from the Q-detector 63 to the adder 83, and the Q-reference settler 81 outputs the reactive power reference signal Q.sub.R to the adder 83. The adder 83 operates a subtraction of the signal Q.sub.F from the signal Q.sub.R and outputs a signal representing the result of the operation to the control compensator 84. The control compensator 84 operates the phase difference angle .phi. and sends it to the phase controller 85. The phase controller 85 receives the phase difference angle .phi. and the system voltage signal from the control compensator 84 and the potential transformer 61, respectively, and outputs the ON-period instruction signals to the inverter 10 so that the phase difference angle between the inverter output voltage V.sub.IN and the system voltage V.sub.SY .phi..
In FIG. 1, when the control compensator 84 outputs the predetermined phase difference angle .phi., the inverter 10 outputs a reactive power Q predetermined in the Q-reference settler 81 to the system power source 100. When the reactive power signal Q.sub.F output from the Q-detector 63 is the same as the reactive power reference signal Q.sub.R output from the Q-referenced settler 81, the active power P corresponding to the phase difference angle .phi. is supplied from the system power source 100 to the inverter main circuit 1 in accordance with formula (1). That is, the phase difference angle .phi. is a negative value, and the active power P corresponding to the phase difference angle .phi. is equal to a loss portion of the inverter main circuit 1 because the reactive power settled in the Q-reference settler 81 cannot be output from the inverter 10 to the system power source 100, as hereinafter described in detail.
When the active power P corresponding to the phase difference angle .phi. is larger than the loss portion of the inverter main circuit 1, the portion of the active power beyond the loss portion of the inverter main circuit 1 charges the DC capacitor 20, thereby increasing the DC voltage V.sub.DC between both the ends of the DC capacitor 20. The relation between the AC voltage V.sub.IN output from the inverter 10 and the DC voltage V.sub.DC of the DC capacitor 20 is expressed in the following formula: EQU V.sub.IN =k.multidot.V.sub.DC ( 4)
wherein k is a fixed number. Therefore, when the DC voltage V.sub.DC between both the ends of the DC capacitor 20 increases, an effective value V.sub.IN of the inverter AC output voltage increases. Thus, since the reactive power Q output from the inverter 10 is larger than that predetermined in the Q-reference settler 81, the result of the subtraction of the reactive power signal Q.sub.R from the reactive power reference signal Q.sub.F in the adder 83 becomes a positive value. Hence, the input signal of the control compensator 84 has a positive value, and the phase difference angle .phi. output from the control compensator 84 becomes a positive value. When the phase difference angle .phi. is the positive value, the active power P corresponding to the positive phase difference angle .phi. is fed from the inverter 10 to the system power source 100 in accordance with formula (1), and thus the DC voltage V.sub.DC between both the ends of the DC capacitor 20 is reduced. Accordingly, the effective value V.sub.IN of the inverter output voltage decreases in accordance with formula (4 ), and then the reactive power Q generated from the inverter 10 is reduced in accordance with formula (2). As described above, when the active power P corresponding to the phase difference angle .phi. is not equal to the loss of the inverter main circuit 1, the reactive power corresponding to the reactive power predetermined in the Q-reference settler 81 can be output from the inverter 10.
In the conventional reactive power controller of FIG. 1, three-phase AC voltages of the system power source 100 are seldom balanced or often unbalanced. When the three-phase AC voltages of the system power source 100 are unbalanced, the DC voltage V.sub.D between both the ends of the DC capacitor 20 pulsates with the same frequency as that of a fundamental wave of the system power source 100. Hence, the amplitude of any phase of the output voltages of the inverter 10 differs in its positive and negative sides. In FIG. 3, there are shown three wave forms when the DC voltage V.sub.D between both the ends of the DC capacitor 20 pulsates with the same frequency as the fundamental wave of the system power source 100. That is, FIG. 3a shows one cycle of a pulsation of the DC voltage V.sub.D of the inverter 10 corresponding to the pulsation of the inverter output voltage in one of the three phases. FIG. 3b shows one cycle of a positive component of the inverter output voltage of the one phase, and FIG. 3c shows one cycle of a negative component of the inverter output voltage of the one phase. As shown in FIGS. 3b and 3c, the amplitudes of the positive and negative components of the inverter output voltage of the one phase are unbalanced, and hence the unbalance between the positive and negative components of the inverter output voltage causes a DC current component contained in the AC current flowing through the inverter side of the reactor 30 and the transformer 40 of FIG. 1. This DC current component biases the magnetic fields of the reactor 30 and the transformer 40 and thus causes an overcurrent in the inverter output AC current of the corresponding one phase, with the result of disabling the operation of the inverter 10. This phenomenon cannot be prevented by only controlling the reactive power signal Q.sub.F supplied to the system power source 100 to be coincident with the reactive power reference signal Q.sub.R output from the Q-reference settler 81 by the control compensator 84.